Problem: $-9rs + 10rt + 3r - 10 = 7s + 3$ Solve for $r$.
Answer: Combine constant terms on the right. $-9rs + 10rt + 3r - {10} = 7s + {3}$ $-9rs + 10rt + 3r = 7s + {13}$ Notice that all the terms on the left-hand side of the equation have $r$ in them. $-9{r}s + 10{r}t + 3{r} = 7s + 13$ Factor out the $r$ ${r} \cdot \left( -9s + 10t + 3 \right) = 7s + 13$ Isolate the $r$ $r \cdot \left( -{9s + 10t + 3} \right) = 7s + 13$ $r = \dfrac{ 7s + 13 }{ -{9s + 10t + 3} }$